Aluminum alloy structural part, method for producing the same, and aluminum alloy sheet

ABSTRACT

Disclosed are a structural part obtained from a 6000 series aluminum alloy sheet as a shaping raw material and having an improved crash performance; and a method for producing the sheet. For the sheet, a 6000 series aluminum alloy sheet is used which has a specified composition and is produced in the usual way. Even when this sheet is used, strain is given at a high level thereto by a cold work, thereby heightening the average dislocation density of a surface of the resultant structural part, which has been artificially aged. This density is measured by X-ray diffraction. Thus, the structural part is improved in strength and in crash performance, which is estimated in a VDA bending test, when the automobile collides.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a structural part which is obtainedfrom a 6000 series aluminum alloy sheet (rolled sheet) as a shaping rawmaterial and is excellent in crash performance (shock absorptionperformance); a method for producing the member; and an aluminum alloysheet.

Description of Related Art

In recent years, social needs of making automotive bodies lighter havebeen further increasing in light of concern for the global environmentand others. In order to respond to the needs, instead of steel materialfor steel sheets or others that have been hitherto used, aluminum alloymaterial has been used in, out of automotive body parts, panels (outerpanels such as a hood, doors and a roof, and inner panels), reinforcingmaterial such as a bumper reinforcement (bumper R/F) and door beams, andother parts.

In order to make automotive bodies lighter, it becomes necessary toenlarge the application of aluminum alloy material to, out of automotivemembers, also automotive structural parts contributing particularly tothe weight saving thereof, such as side members or such members, frames,and pillars. However, it is necessary to give these automotivestructural parts a new crash performance (crash resistance or crashcharacteristic), which results in a further weight saving of raw sheetsfor the structural parts, in a high shock absorption performance of themembers when the automotive body collides, and in the protection ofvehicle passengers.

About high strength reinforcing materials out of automotive structuralparts as described above, it has been already popular to use, as rawmaterial thereof, extruded shapes each produced by hot-extruding an JISor AA7000 series aluminum alloy. In the meantime, about large-sizedstructural parts such as frames or pillars, it is preferred to use, asraw material thereof, rolled sheets each produced by an ordinary method,such as a method of homogenizing an ingot, and then hot-rolling theworkpiece, or optionally cold-rolling the hot-rolled workpiece further.However, the 7000 series aluminum alloy is not easily produced into anyrolled sheet because of high-level alloying thereof. Thus, the rolledsheets have not been very much put into practical use.

For this reason, as alloys for rolled sheets that are produced by anordinary rolling method (in the usual way), attention has been paid toJIS or AA6000 series aluminum alloys, which are Al—Mg—Si aluminum alloysproduced easily because these alloys are lower-level alloying metalsthan the 7000 series alloys.

Sheets of the 6000 aluminum alloys have been already used as large-sizedautomotive body panels (outer panels such as hoods, fenders, doors, aroof and a trunk lid; and inner panels). Thus, in order for the alloysheets to have both of press formability and BH response (bakehardenability), which are required for these large-sized automotive bodypanels, or to be improved in both the properties, many suggestions havebeen made about metallurgically remedial measures about, e.g., componentcomposition or microstructure.

For reinforcing materials as described above and others, 6000 seriesaluminum alloy extruded shapes have been hitherto suggested and put intopractical use. However, for automotive structural parts, few examples ofan aluminum alloy rolled sheet have been suggested.

Only Patent Literature 1 (JP 2001-294965 A) and other literaturessuggest 6000 series aluminum alloy sheets in which controls are madeabout the size and the aspect ratio of crystal grains that are relatedto an aluminum alloy rolled sheet microstructure, whereby after thesheets are artificially aged, the sheets are improved to have a yieldstrength of 230 MPa or more and are heightened in crash performance.

In the meantime, as is well known, about means for controlling thecomposition or the microstructure of a 6000 series aluminum alloy rawsheet in order to improve the formability or strength properties of thissheet for a panel as described above, many suggestions have beenhitherto made about controls of the grain diameter of crystal grains,controls of the texture, and controls of clusters of atoms in the sheet.

These microstructure-controlling means also include various means ofcontrolling the amount of Mg, Si or Cu solid-solutionized in the alloysheet, and of controlling the dislocation density thereof.

For example, Patent Literature 2 (JP 2008-174797 A) suggests that inorder to gain, for a panel, a 6000 series aluminum alloy sheet which isexcellent in stability at ordinary temperature and is not be easilylowered in BH response and other material-qualities by natural aging atroom temperature, the solute Si amount and the solute Mg amount thereinare set into the range of 0.55 to 0.80% by mass and that of 0.35 to0.60% by mass, respectively, and the ratio of the solute Si amount tothe solute Mg amount is set into the range of 1.1 to 2.

Patent Literature 3 (JP 2008-266684 A) also suggests, for a panel asdescribed above, a 6000 series aluminum alloy sheet that is for beingwarm-formed and is excellent in BH response in which the amount ofsolute Cu that is measured by a residue extracting method is set intothe range of 0.01 to 0.7% and further the average crystal grain diameteris set into the range of 10 to 50 μm.

Furthermore, Non-Patent Literature 1 (Journal of Japan Institute ofMetals and Materials, vol. 75, No. 5 (2011), pp. 283-290, “Experimentaland Computationally Scientific Research on Competitive PrecipitationObserved in Al—Mg—Si alloy Having High Dislocation Density and UltrafineGrain Microstructure” Tetsuya Masuda, Shoichi Hirosawa, Zenji Hotta, andKenji Matsuda) suggests that the following are forecasted in order tomake a 6000 series aluminum alloy sheet higher in strength:microstructural parameters (dislocation density and crystal graindiameter) for combining dislocation strengthening or crystal grainrefinement strengthening optimally with precipitation strengthening.

This literature states that: about samples each obtained by subjecting a6000 series aluminum alloy sheet to cold rolling, or HPT working, whichis a high-pressure torsion method, the dislocation densities thereofhave been inspected. As a result, samples not subjected to the workinghave a dislocation density of about 10¹¹ m⁻², and the samplescold-rolled at a rolling ratio of 30% (corresponding strain: 0.36) havea dislocation density of about 10¹⁴ m⁻². Measurements of the dislocationdensities are made by an equal thickness interference method in which 5visual fields in a 100000-magnification TEM photograph of each of thesamples are used in an intersection analysis manner.

According to this Non-Patent Literature 1, an inspection has been madeabout conventional technique reports each stating that when themicrostructure of a 6000 series aluminum alloy sheet is controlled fordislocation strengthening or crystal grain refinement strengthening, theartificial age-hardenability of the sheet is frequently restrained in asubsequent artificial aging of the sheet, so it is difficult to attainconsistency between the two strengthening mechanisms.

Results of the test demonstrate that: as the artificial aging periodelapses, the non-worked alloy sheets and the cold-rolled alloy sheetsare increased in hardness; about the non-worked alloy sheets, the valueobtained by subtracting the hardness thereof before the artificial agingtreatment from the peak hardness thereof is 75 HV while about thecold-rolled alloy sheets, the value is 43 HV, which has become reverselysmaller; thus, the cold rolling makes the artificial age-hardenabilitylow. The literature states that as the aging period elapses, the HPTalloy sheets are monotonously decreased in hardness not to exhibit anaging hardening behavior.

Structural parts of automobiles and others in which the presentinvention is to be used are required to have properties peculiar to theuse, for example, are required to be further heightened in strength, andto be newly caused to have a shock absorption performance, that is,crash resistance when the automotive body collides.

For example, according to a matter that collision safety standards ofautomobiles have been raised (or made severer) in recent years, inEurope and others, structural parts of an automobile, such as frames andpillars, have been required to satisfy crash performance (crashresistance or shock absorption performance) when the automobilecollides, this property being evaluated in a “VDA 238.100 plate bendingtest for metallic materials (hereinafter referred to as a VDA bendingtest)”, which is standardized by Verband der Automobilindustrie e.V.(VDA).

Against such severe safety standards, structural parts of any automobilethat are obtained from 6000 series aluminum alloy sheets as shaping rawmaterials, these sheets being produced by an ordinary rolling method,are short in the following property when the automobile collides: crashperformance that has been obtained by making the alloy sheets higher instrength. As means for causing such automotive structural parts, whichare obtained from 6000 series aluminum alloy sheets as shaping rawmaterials, to satisfy the crash performance, an effective means has notyet been clear even when the existence of the above-mentioned non-patentliterature is known. Thus, there remains a room for realizing such ameans.

SUMMARY OF THE INVENTION

In light of such a situation, an object of the present invention is toprovide a structural part which is obtained from a 6000 series aluminumalloy sheet as a shaping raw material and is improved in crashperformance; a method for producing the member; and an aluminum alloysheet.

For attaining this object, a subject matter of the aluminum alloystructural part of the present invention excellent in crash performanceis a member, comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, thepercent symbols each representing % by mass, and Al and inevitableimpurities as the balance of the member; and this structural part havingan average dislocation density of 3.0×10¹⁴ to 8.0×10¹⁴ m⁻², the densitybeing measured by X-ray diffraction of the surface.

For attaining this object, a subject matter of the method of theinvention for producing an aluminum alloy structural part excellent incrash performance is a method including: applying homogenization to analuminum alloy ingot comprising Mg: 0.30 to 1.5%©, and Si: 0.50 to 1.5%,the percent symbols each representing % by mass, and Al and inevitableimpurities as the balance of the ingot, and subsequently rolling theingot into a sheet; subjecting this sheet further to solutionizing andquenching treatments, and subsequently cold-working the treated sheet tobe formed into a structural part while giving a strain of 5 to 20% tothe sheet; thereby adjusting the artificially aged structural part tohave a dislocation density of 3.0×10¹⁴ to 8.0×10¹⁴ m⁻², the densitybeing measured by X-ray diffraction of the surface.

For attaining this object, a subject matter of the aluminum alloy sheetof the invention excellent in crash performance is a sheet for astructural part comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, thepercent symbols each representing % by mass, and Al and inevitableimpurities as the balance of the sheet; and the following sheet having,as a microstructure, an average dislocation density of 3.0×10¹⁴ to8.0×10¹⁴ m⁻², the density being measured by X-ray diffraction of thesurface: a surface of a sheet which is obtained, for simulating use ofthe structural part, by subjecting the sheet to solutionizing treatmentof keeping the sheet at 550° C. for 30 seconds, water quenching thesheet immediately down to room temperature at an average cooing rate of30° C./s, subjecting the sheet, immediately after the quenching, to apre-aging treatment at 100° C. for 5 hours, giving a strain of 10%,after the treatment, to the sheet through a tensile tester and furtheraging the sheet artificially at 210° C. for 30 minutes.

In the present invention, a cold work, such as press forming to beapplied to a structural part of an automobile or some other, is used togive (add) a large quantity of strain beforehand to a raw sheet (rolledsheet) subjected to a tempering treatment, such as solutionizingtreatment, thereby making a surface of the formed structural part higherin dislocation density than in the prior art.

Such a structural part, the dislocation density of which has been madehigh, is artificially aged to cause the finally obtained structuralpart, which is to be used, to exhibit a high crash performance ofshowing a high 0.2% yield strength of 250 MPa or more and a bendingangle of 90° or more according to a VDA bending test.

Detailed mechanism of the exhibition (reasons therefor) have not yetbeen clear. However, it is presumed that: the structural part surface ismade higher in dislocation density level than any raw sheet orstructural part used for an ordinary panel, thereby increasingremarkably an effect of preventing dislocation-movement when thestructural part is collapsed to be deformed, for example, when theautomobile undergoes a collision accident; thus, the structural part isimproved in balance between strength and crash performance.

This effect is increased also by an increase in the Cu amountsolid-solutionized in the structural part, so that this member isincreased in solute strengthening level to be heightened in strength andbe further restrained from undergoing dislocation-localization whencollapsed to be deformed, thereby being also improved in crashperformance.

The present invention makes it possible to improve a 6000 seriesaluminum alloy raw rolled sheet, which has already been standardized asa structural part, in crash performance without changing the compositionand the production process of the rolled sheet largely and withoutlowering the formability and other properties of the aluminum alloy rawrolled sheet.

Moreover, by giving (adding) a large quantity of strain beforehand tothe aluminum alloy raw rolled sheet when the sheet is subjected to pressforming or some other cold work to be made into a structural part, thestructural part can be heightened in dislocation density in a productionprocess of the structural part without increasing the number of stepsfor the cold work.

Thus, the present invention makes it possible to apply a raw rolledsheet to a structural part that is a security member important forautomobiles even when this sheet is a 6000 series aluminum alloy rawrolled sheet used for an ordinary panel.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view illustrating a manner of a VDA bending testfor evaluating the shock absorption performance of a metallic testspecimen.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A structural part referred to in connection with the present inventiondenotes a structural part having a relatively large thickness of about 2to 10 mm to function as a skeleton of, in particular, a transportingmachine such as an automobile or a railroad vehicle, and is strictlydistinguished from any large-sized body panel having a relatively smallthickness less than 2 mm, such as an outer or inner panel.

The dislocation density of a surface of the structural part, which isspecified in the present invention, is decreased by an artificial agingtreatment of the structural part, such as paint-baking treatmentthereof.

Accordingly, in order to ensure the crash performance of the structuralpart when the member is used, the dislocation density stipulated in thepresent invention is specified about the structural part after themember is artificially aged.

An aluminum alloy raw sheet referred to in connection with the presentinvention denotes an aluminum alloy raw sheet that is a hot-rolledsheet, a cold-rolled sheet or any other rolled sheet subjected to atempering (T4) treatment, such as solutionizing treatment or quenchingtreatment, and that is a sheet which has not yet been formed into anautomotive structural part to be used. In the following description,aluminum may be represented also as Al. Hereinafter, embodiments of thepresent invention will be specifically described in accordance with eachrequirement of the invention.

Aluminum Alloy Composition:

Initially, a description is made about not only the chemical componentcomposition of an aluminum alloy sheet in the present invention, butalso reasons why elements to be used therein and the respective contentsby percentage of the elements are limited. The percent symbol(s) forshowing the content of each of the elements (each) represent(s) % bymass.

The chemical component composition of the aluminum alloy sheet in thepresent invention functions as a presupposition for a purpose that astructural part obtained finally by aging a 6000 series aluminum alloyartificially, or a raw sheet to which strain or heat treatment is givenor applied in such a manner that this raw sheet simulates the structuralpart described just above can satisfy a specified dislocation densityand further gain a required strength and crash performance, and canpreferably have, together therewith, formability into structural parts.

From these viewpoints, the chemical component composition of thealuminum alloy sheet in the present invention is rendered a compositionincluding Mg: 0.30 to 1.5% and Si: 0.50 to 1.5%, and Al and inevitableimpurities as the balance of the composition.

In order to improve the sheet in strength, this composition may furtheroptionally include Cu in a proportion of 0.05 to 1.0%, or solute Cu in asolution separated from the alloy sheet by a residue extracting methodwith hot phenol in a proportion of 0.05 to 1.0% of the solution.

Moreover, in order to improve the strength, the composition mayoptionally include one or more of the following: Mn: 0.05 to 0.5%, Zr:0.02 to 0.20%, and Cr: 0.02 to 0.15%.

Furthermore, in order to improve the strength, each of theabove-mentioned compositions may optionally include one or more of thefollowing: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%.

The percent symbol(s) for showing the content of each of the elements(each) represent(s) % by mass.

Si: 0.50 to 1.5%

Si is combined with Mg to produce a Mg—Si precipitate, which contributesto solute strengthening, and an improvement in the strength of thestructural part when the aluminum alloy is subjected to an artificialaging treatment such as paint-bake treatment, thereby exhibiting aginghardenability. Thus, this element is an element essential for causingthis structural part to gain a strength (yield strength) necessary forautomobiles and others.

If the Si content by percentage is too small, the solute Si amount isdecreased before the paint-bake treatment (before the artificial agingtreatment) so that the amount of the produced Mg—Si precipitate isinsufficient. Thus, the structural part is remarkably lowered in BHresponse to be short in strength or crash performance.

In the meantime, if the Si content by percentage is too large, coarsecrystallized and precipitated products are produced so that the aluminumalloy is lowered in ductility to be cracked when rolled. Thus, the Sicontent is set into a range from 0.50 to 1.5%, preferably from 0.70 to1.5%.

Mg: 0.30 to 1.5%

Mg is combined with Si to produce a Mg—Si precipitate, which contributesto solute strengthening, and an improvement in the strength of thestructural part when the aluminum alloy is subjected to an artificialaging treatment such as paint-bake treatment, thereby exhibiting aginghardenability. Thus, this element is an element essential for causingthis structural part to gain a yield strength necessary for automobilesand others.

If the Mg content by percentage is too small, the solute Mg amount isdecreased before the artificial aging treatment so that the quantity ofthe produced Mg—Si precipitate is insufficient. Thus, the structuralpart is remarkably lowered in BH response to be short in strength orcrash performance.

In the meantime, if the Mg content by percentage is too large, thealuminum alloy easily undergoes the formation of shear zones thereinwhen cold-rolled, to be cracked in the rolling. Thus, the Mg content isset into a range from 0.3 to 1.5%, preferably from 0.7 to 1.5%©.

Cu: 0.05 to 1.0%

Cu makes the structural part high in strength by solute strengthening,and further improves the crash performance by a restraint ofdislocation-localization when the member is collapsed to be deformed. Ifthe Cu content by percentage is too small, this advantageous effect issmall. If the content is too large, the advantageous effect issaturated, and the corrosion resistance and others are converselydeteriorated. Thus, Cu is optionally incorporated in a range from 0.05to 1.0% into the aluminum alloy.

Solute Cu Amount: 0.05 to 1.0%

When Cu is incorporated to be caused to ensure (exhibit) thestrength-heightening or the crash-performance-improving effect by thesolute strengthening of Cu, the solute Cu amount in a solution separatedfrom the structural part by a residue extracting method with hot phenolis set into a range from 0.05 to 1.0% of the solution. As the solute Cuamount is larger, the structural part is made better in workhardenability, smaller in yield ratio and larger in elongation to beimproved in crash performance.

If the solute Cu amount is less than 0.05% regardless of the Cu contentby percentage, the advantageous effect thereof is insufficient. Theupper limit of the solute Cu amount is substantially equal to that ofthe added amount of Cu.

Mn: 0.05 to 0.5%; Zr: 0.02 to 0.20%; and Cr: 0.02 to 0.15%

One or more of Mn, Zr and Cr may be optionally incorporated into thealuminum alloy, as the same advantageous-effect element(s) for makingcrystal grains in the ingot or raw sheet finer, to contribute to animprovement of the finally obtained structural part in strength.

These elements are present in a dispersed particle form to contribute tocrystal grain refinement to produce also an advantageous effect ofimproving the raw sheet in formability. If the content by percentage ofeach of the elements is too small, the strength- orformability-improving effect based on the crystal grain refinement isinsufficient. If the content is too large, coarse compound grains areproduced to deteriorate the aluminum alloy in ductility.

Thus, when one or more of Mn, Zr and Cr are optionally incorporated, theincorporation is attained as follows: Mn: 0.05 to 0.5%, Zr: 0.02 to0.20%, and/or Cr: 0.02 to 0.15%.

Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%

One or more of Ag, Sn and Sc may be optionally incorporated into thealuminum alloy as the same advantageous-effect element(s) for improvingthe strength thereof.

Ag causes an aging precipitate contributing to an improvement of thestructural part in strength to be densely and minutely precipitated byan artificial aging treatment after the raw sheet is formed into thestructural part, thereby producing an advantageous effect of promotingthe enhancement of the strength. Thus, as required, Ag is optionallyincorporated. If the Ag content is less than 0.01%, thestrength-improving effect is small. If the Ag content is too large, thealuminum alloy is conversely lowered in various properties such asrollability and weldability, and further the strength-improvingadvantageous effect is also saturated merely to increase costs. Thus,when Ag is optionally incorporated, the Ag content is set into a rangefrom 0.01 to 0.2%.

Sn has an advantageous effect of restraining the production of clustersat room temperature to keep an excellent formability or workability ofthe raw sheet after a solutionizing/quenching treatments thereof, andfurther improving the strength when the sheet is subsequently subjectedto an artificial aging treatment, such as paint-bake treatment. Sn istherefore an element essential for giving the structural part a yieldstrength and crash performance necessary for structural parts ofautomobiles. If the Sn content is less than 0.001%, the advantageouseffects are small. If the content is more than 0.1%, the advantageouseffects are saturated, and the aluminum alloy conversely undergoes hotbrittleness to be remarkably deteriorated in hot workability (heatstretchability). Thus, when Sn is optionally incorporated, the Sncontent is set into a range from 0.001 to 0.1%.

Sc makes crystal grains in the ingot and the finally obtained productfine to contribute to an improvement in the strength thereof. Moreover,Sc is dispersed in a dispersed particle form to contribute to crystalgrain refinement to improve the raw sheet also in formability. If the Sccontent by percentage is too small, these advantageous effects areshort. If the content is too large, coarse compound grains are producedto deteriorate the aluminum alloy in ductility. Thus, when Sc isoptionally incorporated, Sc is incorporated in a range from 0.02 to0.1%.

Other Elements:

Elements other than the above-mentioned elements, for example, Ti, B,Fe, Zn and V are inevitable impurities. The aluminum alloy may containeach of these elements in a content range specified in the JIS Standardand others for 6000 series alloys.

Dislocation Density:

Under the presupposition of the above-mentioned alloy composition, aboutthe microstructure of each surface (microstructure obtained by observingthe surface) of the structural part subjected to an artificial agingtreatment, or each surface of a raw sheet to which strain or heattreatment is given or applied in such a manner that the raw sheetsimulates this structural part, the dislocation density measured byX-ray diffraction is set into a range from 3.0×10¹⁴ to 8.0×1024 m⁻²,preferably from 4.0×10¹⁴ to 8.0×10¹⁴ m⁻² on average.

About the surface of the artificially aged structural part, or thesurface of the raw sheet to which the strain or heat treatment is givenor applied in such a manner that the raw sheet simulates this structuralpart, the dislocation density is set into the specified range, wherebythe structural part can have a 0.2% yield strength of 250 MPa or more,and further have such a crash performance that a bending angle of 90° ormore is obtained according to a VDA bending test thereof.

It can be presumed that: the dislocation density level of the structuralpart surface is made higher than that of any surface of a raw sheet foran ordinary panel, or a panel obtained from this raw sheet as rawmaterial, whereby the structural part produces a remarkably increasedeffect of hindering dislocation-movement when collapsed to be deformed,for example, when the automobile meets with a collision accident; andthus, the member is improved in balance between strength and crashperformance.

In connection with this point, if the dislocation density is less than3.0×10¹⁴ m⁻² to be too small, the structural part is equivalent toconventional members (such as panels) not to exhibit required strengthand crash performance.

In the meantime, if the dislocation density exceeds 8.0×10¹⁴ m⁻² onaverage to be too large, the aluminum alloy is lowered in elongation tobe conversely lowered in crash performance.

In the present invention, when a raw sheet (rolled sheet) subjected to atempering treatment, such as solutionizing treatment, is subjected to acold work, such as press forming, so as to be made into a structuralpart, or before or after this press forming, the sheet is furthercold-worked to add (give) strain beforehand to the structural part. Inthis way, the dislocation density of any surface of the formedstructural part is heightened into the above-mentioned specified range.

For reference, when an aluminum alloy raw sheet is made into a panel byan ordinary press forming, strain given thereto is generally a smallvalue less than 5%. Consequently, the dislocation density of any surfaceof a structural part obtained from the raw sheet cannot be adjusted intothe range specified in the present invention after the member isartificially aged.

Considering conditions for an artificial aging treatment such as anordinary paint-baking treatment at high temperature for a short period,in order to set the dislocation density of the structural part surfaceinto the range specified in the present invention, it is necessary toset, into 5% or more, preferably 10% or more, the strain given to thestructural part by press forming into this structural part, or thestrain given thereto by a combination of the press forming with coldwork performed before or after the press forming.

However, if the given strain is more than 20%, the dislocation densityof the structural part surface may exceed 8.0×10¹⁴ m⁻² on average tobecome too large under conditions for an artificial aging treatment ofthe structural part, such as an ordinary paint-baking treatment thereofat high temperature for a short period. Thus, the structural part may belowered in elongation to be conversely lowered in crash performance.

Considering that strain is given to the raw sheet when the sheet ispress-formed into a structural part, it is actually difficult to givestrain or dislocation in a quantity more than described in the above.

Accordingly, in order to control the average dislocation density intothe range specified in the present invention under conditions for theartificial aging treatment, such as the high-temperature andshort-period ordinary paint-baking treatment, it is preferred to selectan optimal strain to be given from a range from 5 to 20%, preferablyfrom 10 to 20% while the artificial aging conditions are alsoconsidered.

For reference, the Non-Patent Literature 1 suggests dislocation(density) strengthening for making a 6000 series aluminum alloy sheethigher in strength. The 6000 series aluminum alloy sheet is artificiallyaged through cold rolling, or HPT working, which is a high-pressuretorsion method.

However, as described in the conventional technique reports, the testresults support only the fact that even when the 6000 series aluminumalloy sheet is dislocation-strengthened (increased in dislocationdensity), the sheet is restrained from having aging-hardenability whilesubsequently artificially aged. The literature never describes anyrelationship between this dislocation density and the crash performanceof the structural part obtained by formation of the sheet and artificialaging treatment thereof.

The microstructure or mechanical properties of this (original)structural part subjected to the artificial aging treatment can beevaluated by examining microstructure or mechanical properties of thefollowing even when a raw sheet therefor is actually subjected toformation into a structural part followed by an artificial agingtreatment: the microstructure and mechanical properties of a productobtained, for simulating this original structural part, by subjecting a6000 series aluminum alloy raw sheet subjected to tempering treatments,such as solutionizing and quenching treatment, to a cold work for givingstrain thereto, such as press forming, followed by an artificial agingtreatment.

About preferred treatment conditions for simulating this originalstructural part, in order to simulate a specific usage of the originalstructural part, a 6000 series raw aluminum alloy sheet is subjected tosolutionizing treatment at a temperature selected from the range of atemperature of 550° C. to the melt temperature of the sheet (bothinclusive), this range being one out of preferred producing conditionsthat will be also described later, for about 0.1 sec to several tens ofseconds in a continuous furnace, or for about several tens of minutes ina batch furnace; immediately, the sheet is rapidly cooled to roomtemperature at an average cooling rate of 20° C./sec or more and, issubjected, immediately after the cooling, to a pre-aging treatment ofkeeping the sheet at 60 to 120° C. for 2 to 10 hours; and then a strainof 10 to 20% is given to the resultant sheet through a tensile tester,and subsequently the sheet is further artificially aged at 210 to 270°C. for 10 to 30 minutes. By examining the microstructure and mechanicalproperties of the product obtained in this way, the original structuralpart can be evaluated with a high correlation and a goodreproducibility.

In order to make this reproducibility stricter in the aluminum alloysheet of the present invention, specific treatment conditions forsimulating the original structural part are rendered one-pointconditions of subjecting an aluminum alloy sheet to a solutionizingtreatment of keeping the sheet at 550° C. in a batch furnace for 30seconds; immediately water quenching the sheet to room temperature at anaverage cooling rate of 30° C./sec; subjecting the sheet, immediatelyafter the quenching, to a pre-aging treatment at 100° C. for 5 hours;and subsequently giving the resultant sheet a strain of 10% through atensile tester, and further aging this sheet artificially at 210° C. for30 minutes.

Method for Measuring Dislocation Density

As described in the Non-Patent Literature 1 and others, it is widelyused to measure the dislocation density of, e.g., a metallic sheetthrough, e.g., a transmission electron microscope. In the presentinvention, the dislocation density is more easily measured with a betterreproducibility by X-ray diffraction. Regions (cell walls and shearzones) where linear and streak dislocations, out of dislocations, gatherdensely are not easily discriminated through any transmission electronmicroscope, so that the regions cause measurement accidental errors whenthe dislocation density p is analyzed and gained. In contrast, X-raydiffraction produces an advantage that even such dislocations gatheringin large numbers cause only small accidental errors since thedislocation densities p are calculated out from the respectivehalf-value widths of diffraction peaks obtained from individual surfacesof the texture, as will be detailed later.

In the microstructure of a sheet into which dislocations are introducedby adding plastic deformations to the sheet by, e.g., cold rolling or atensile test, lattice strains are generated to centralize thedislocations. Moreover, by the arrangement of the dislocations,low-angle grain boundaries, cell structures, and others are developed.When such dislocations, or domain structures following the dislocationsare caught from the resultant X-ray diffraction pattern, in accordancewith diffraction indexes thereof characteristic breadths and shapes maketheir appearance in the diffraction peaks of the pattern. When theshapes (line profile) of the diffraction peaks are analyzed (lineprofile analysis), the dislocation density can be gained.

Specifically, from a structural part artificially aged or a raw sheetsimulating this structural part, a sample is collected in such a mannerthat surfaces of the member or sheet are to be observing surfaces.Microstructures of the surfaces of the sample are then subjected toX-ray diffraction. The half-value width of a diffraction peak of each ofthe (111), (200), (220), (311), (400), (331), (420) and (422) planes(orientation planes), which are main orientations of the texture of thesurfaces of the structural part, are gained.

As the dislocation density p is higher, the half-value width of thediffraction peak of each of these planes is larger. The surfaces of thestructural part as the sample, which are targets to be measured by X-raydiffraction, may be in the state of the surfaces of the sample notsubjected to any further treatment, or may be washed without beingetched.

Next, from the respective half-value widths of the diffraction peaks ofthese individual surfaces, the lattice strain (crystal strain) a isgained by the Williamson-Hall method. Furthermore, the dislocationdensity p of the sample can be calculated out in accordance with thefollowing expression:

ρ=16.1ε² /b ²

wherein ρ represents the dislocation density; ε, the lattice strain ofthe sample; and b, the magnitude of Burger's vector.

As the magnitude of the Burger's vector, 2.8635×10⁻¹⁰ m is used.

The Williamson-Hall method is a known line profile analyzing method usedwidely to gain the dislocation densities or crystal grain diameters of ametal sample from a relationship between respective half-value widths ofplural diffraction peaks of the sample, and the diffraction anglesthereof. Known is also a series of manners for gaining the dislocationdensities by X-ray diffraction. The dislocation densities obtained bythe manner series for gaining the dislocation densities by X-raydiffraction are generically named the “dislocation density measured byX-ray diffraction” in the present invention.

About the dislocation density of any structural part, 10 samplescollected from arbitrarily-selected sites of the structural part aremeasured, and the resultant dislocation densities are averaged.

Producing Method:

The following describes a preferred method for producing the structuralpart of the present invention. Initially, a preferred method forproducing a raw rolled sheet is described hereinafter in the order ofsteps thereof.

A 6000 series aluminum alloy sheet which is a raw material for thestructural part is a hot-rolled sheet obtained by subjecting an ingot tohomogenization followed by hot rolling, or a cold-rolled sheet obtainedby subjecting the hot-rolled sheet to cold rolling, and is produced in ausual manner of subjecting the hot-rolled sheet or cold-rolled sheetfurther to a tempering treatment such as solutionizing treatment.Specifically, the raw material is an aluminum alloy hot-rolled sheetproduced through ordinary individual producing steps composed ofcasting, homogenization and hot rolling, and having a sheet thickness ofabout 2 to 4 mm; or a cold-rolled sheet obtained by cold-rolling ahot-rolled sheet having a larger thickness and produced through the samesteps into a thickness of about 2 to 4 mm.

The 6000 series aluminum alloy sheet in the present invention may beproduced by an especial producing method or rolling method in whichafter continuous casting into a thin sheet in, e.g., a twin roll manner,the thin sheet is cold-rolled with an omission of any hot rolling, or iswarm-rolled.

Accordingly, the producing method has an advantage that a raw sheet canbe produced without making a large change of 6000 series aluminum alloycompositions standardized already for structural parts as describedabove, and without making a large change of a rolling step in the usualway.

Melting, and Casting:

A raw alloy is initially molten and cast. In the melting and castingsteps, a molten aluminum alloy adjusted into the above-mentioned 6000series component composition range, as the molten raw alloy, is cast byan appropriately selected ordinary melting and casting method such as acontinuous casting method or a semi-continuous casting method (DCcasting method).

Homogenization:

Next, the cast aluminum alloy ingot is subjected to homogenization inthe usual way before subjected to hot rolling. A purpose of thishomogenization is to homogenize the microstructure, that is, to removesegregation inside crystal grains in the ingot microstructure.Conditions for this homogenization are appropriately selected from therange of 500° C. or higher and less than the melting point of the alloy,and the holding period range of 2 hours or longer.

Hot Rolling:

The sheet is then hot-rolled. Under a condition that the startingtemperature of the hot rolling is higher than the solid phase linetemperature of the alloy, burning is caused not to conduct the hotrolling itself easily. If the hot rolling starting temperature is lowerthan 350° C., an excessively high load is generated in the hot rollingnot to conduct the hot rolling itself easily. Thus, the hot rolling isperformed at a hot rolling starting temperature selected from the rangeof 350° C. to the solid phase line temperature to produce a hot-rolledsheet having a thickness of about 2 to 10 mm. This hot-rolled sheet isnot necessarily annealed before cold-rolled; however, the sheet may beannealed.

The hot rolling of the ingot subjected to the homogenization is composedof a rough rolling step of the ingot (slab) and a finish rolling step.In these rough and finish rolling steps, a rolling machine of, e.g., areverse type or a tandem type is appropriately used.

During the hot rolling from the start of the hot rough rolling to theend thereof, it is preferred to keep the solute amounts of Si and Mgsurely without lowering the temperature to 450° C. or lower. If thelowest temperature of the rough rolled sheet in the middle of therolling path is lowered to 450° C. or lower, for example, by making therolling period long, one or more compound precipitate easily. Thus, evenwhen strain is given thereto before the sheet is artificially aged, thedislocation density may not be sufficiently increased. Moreover, thepossibility is great that the solute Cu amount is also lowered.

After the hot rough rolling, the sheet is subjected to hot finishrolling the end temperature of which is preferably set into the range of300 to 360° C. If the end temperature of the hot finish rolling is lowerthan 300° C. to be too low, the rolling load becomes high to lower theproducing performance of this method. In the meantime, in the case ofheightening the end temperature of the hot finish rolling to make thealloy into a recrystallized phase without leaving a large quantity ofthe deformed microstructure, coarse transition-element-dispersedparticles may probably precipitate if the end temperature is higher than360° C.

From the temperature of the material (sheet) just after the end of thehot finish rolling to a material temperature of 150° C., the averagecooling rate is controlled into at lowest 5° C./hour or more by forciblecooling using, e.g., fans. If this average cooling rate is less than 5°C./hour, the quantity of a precipitate produced during the coolingbecomes large. Thus, even when strain is given to the sheet before thesheet is artificially aged, the dislocation density does not increasesufficiently. Moreover, the solute Cu amount is decreased in theresultant product sheet.

It is therefore preferred that the average cooling rate is larger justafter the end of the hot finish rolling. The rate is set to at lowest 5°C./hour or more, preferably to 8° C./hour.

For reference, according to any ordinary hot finish rolling, after thisrolling, the resultant sheet is wound into the form of a coil. Thus,when the coil diameter is an ordinary diameter, the average cooling rateaccording to natural cooling just after the end of the hot finish rolleasily turns into less than 5° C./hour as far as the rolled sheet is notforcibly cooled by, e.g., fans.

When the resultant hot-rolled sheet is further cold-rolled, the sheetdoes not need to be annealed before the cold rolling. However, theannealing may be performed.

Cold Rolling:

The sheet is then cold-rolled. In the cold rolling, the hot-rolled sheetis cold-rolled to produce a cold-rolled sheet (that may be in the formof a coil) having a desired final thickness. In order to make thecrystal grain finer, the cold rolling ratio is desirably 30% or more.Moreover, to attain the same purpose that the above-mentioned annealingdoes, the hot-rolled sheet may be subjected to intermediate annealing inthe middle of the cold rolling path.

Solutionizing and Quenching Treatments:

After the cold rolling, the rolled sheet is subjected to solutionizingtreatment followed by quenching treatment down to room temperature. Forthe solutionizing and quenching treatments, an ordinary continuous heattreatment line may be used. In order for the treated sheet to gain asufficient solute amount of each of Mg, Si, and other elements, it ispreferred to perform the solutionizing treatment at the moltentemperature of the sheet or lower, followed by the quenching down toroom temperature at an average cooling rate of 20° C./second or more. Ifthe solutionizing treatment temperature is lower than 550° C., compoundsof Mg—Si and others that have been produced before this solutionizingtreatment are insufficiently re-solid-solutionized so that the soluteamounts of Mg and Si are lowered.

If the average cooling rate is less than 20° C./second, the possibilitybecomes great that during the cooling, Mg—Si precipitates are producedto lower the solute amounts of Mg and Si so that sufficient soluteamounts of Mg and Si cannot be ensured. In order to ensure the coolingrate in the quenching, a cooling means is selected from fans and otherair-cooling means or manners, and mist, spraying, immersion and otherwater-cooling means or manners, as well as cooling conditions areselected.

Pre-Aging Treatment: Reheating Treatment:

After the solutionizing treatment followed by the quenching treatment,resulting in the cooling of the sheet to room temperature in this way,it is preferred to subject the cold-rolled sheet to pre-aging treatment(reheating treatment) within one hour of the cooling end. If the roomtemperature holding period from the end of the quenching down to roomtemperature to the pre-aging start (heating start) is too long, Mg—Siclusters rich in Si amount are unfavorably produced by natural aging atroom temperature, so that an increase cannot easily be made in theamount of Mg—Si clusters good in balance between the Mg and Siproportions. It is therefore more preferred that this room temperatureholding period is shorter. The solutionizing and quenching treatments,and the reheating treatment may be continuously conducted without havingany time lag substantially therebetween. The lower limit of the periodis not particularly specified.

About this pre-aging treatment, a holding period at 60 to 120° C. isadjusted preferably into the range of 2 to 40 hours both inclusive. Inthis case, Mg—Si clusters good in balance between the Mg and Siproportions are produced.

If the pre-aging temperature is lower than 60° C., or the holding periodis shorter than 2 hours, the same results as in the case of notconducting this pre-aging are produced to restrain the production ofMg—Si clusters rich in Si amount. Thus, the Mg—Si clusters good inbalance between the Mg and Si proportions are not easily increased inquantity, so that the alloy sheet is easily lowered in yield strengthafter paint-baked.

In the meantime, if the pre-aging temperature is higher than 120° C., orthe holding period is longer than 40 hours, the quantity of producedprecipitation nuclei is too large so that the alloy sheet is too high instrength when subjected to bending work before the bake-painting. Thus,the sheet is easily deteriorated in bendability.

Production of Structural Part Strain Addition:

The raw sheet subjected to these tempering treatments (T4) is formedinto a product, such as a side member or such a member, a frame, apillar or any other structural part, mainly by press forming.

At this time, the raw sheet is formed into a structural part while astrain of 5 to 20% is given thereto by cold work. In addition thereto,the resultant structural part is artificially aged, thereby making itpossible to set, into the range of 3.0×10¹⁴ or 8.0×10¹⁴ m⁻², thedislocation density of any surface of the structural part subjected tothe artificial aging treatment. This density is measured by X-raydiffraction.

At this time, it is allowable before the artificial aging treatment toapply a cold work for giving the above-mentioned strain beforehand tothe raw sheet when the raw sheet is press-formed into the structuralpart without conducting cold work in a separate step.

In accordance with the shape of the structural part, the strain may beadded thereto by not only the press forming but also a method or meansfor the cold work, such as tension, cold rolling, a leveler or stretch.In this case, the total strain added by the press forming and the coldwork is adjusted into the above-mentioned range.

The strain is made larger than that added at the time of press formingfor producing, e.g., an automotive panel in the usual way, and then thestrain is beforehand added (given) to the alloy sheet before the sheetis artificially aged. In order to adjust the average dislocation densityinto the above-mentioned range of 3.0×10¹⁴ to 8.0×10¹⁴ m⁻², the strainis added thereto in a proportion of 5% or more, preferably of from 10 to20% both inclusive.

As described above, if the strain is less than 5%, this strain, whichdepends on the artificial aging treatment conditions, is not largelydifferent from that given when any conventional press forming or bendingworking is conducted. Thus, the average dislocation density cannot beadjusted to 3.0×10¹⁴ m⁻² or more.

As the strain is larger, the average dislocation density can be madelarger. However, if the strain is more than 20%, the average dislocationdensity exceeds 8.0×10¹⁴ m⁻² so that the resultant structural part isremarkably lowered in elongation to be poor in crash performance.

Artificial Aging Treatment:

The artificial aging treatment of the strain-given structural part or asheet to which a strain of 5 to 20% is given to simulate this structuralpart may be conducted by paint-bake treatment or an ordinary artificialaging treatment (T6 or T7).

Conditions for the heating temperature and the holding period are freelydecided in accordance with, e.g., a desired strength of the structuralpart or the natural-aging-advancing degree thereof at room temperature.When the artificial aging treatment is, for example, a one-stagetreatment, the artificial aging treatment is conducted preferably at aheating temperature of 200 to 270° C. for a holding period of 5 to 30minutes.

If the heating temperature is too low or the holding period is tooshort, the structural part may undergo insufficient aging hardening notto have a strength or crash performance that is a target of the presentinvention. Also if the heating temperature is too high or the holdingperiod is too long, the structural part may undergo over-aging not tohave a strength or crash performance that is a target of the presentinvention.

EXAMPLES

In each of invention examples and comparative examples, as shown inTable 2 described below, one out of variously changed strains was addedto a cold-rolled sheet of a tempering-applied 6000 series aluminum alloyhaving one out of component compositions shown in Table 1 to simulatethe structural part concerned. The resultant was artificially aged, andthen measurements and evaluations were made about the microstructure ofany surface of this artificially-aged test material (the averagedislocation density thereof and the solute amount of Cu therein), andthe strength and the crash performance evaluated in a VDA bending test.The results are shown in Table 2. In the indication of the content bypercentage of each element in Table 1, the symbol “-” shown in anumerical value cell about the element denotes that the content bypercentage is not more than the limit of detection.

The above-mentioned cold-rolled sheet as a raw sheet was specificallyproduced as follows:

An aluminum alloy ingot having the composition shown in Table 1 wasmolten and cast into an ingot by a DC casting method. Subsequently, theingot was subjected to homogenization at a temperature-raising rate of150° C./hour and at a homogenization temperature of 550° C. for aholding period of 3 hours.

Thereafter, hot rough rolling of the workpiece was started at 500 to520° C., and the lowest temperature of the hot rough rolling was set toone out of variously changed temperatures shown in Table 2. Furthermore,the workpiece was subjected to hot finish rolling the end temperature ofwhich was set into the range of 300 to 350° C. to produce a hot-rolledsheet of 4.0 mm thickness.

At this time, the average cooling rate (° C./hour) from the material(sheet) temperature just after the end of the hot finish rolling to amaterial temperature of 150° C. was set to one out of variously changedvalues as shown in Table 2.

This hot-rolled sheet was cold-rolled at a rolling ratio of 50% withoutbeing subjected to heat treatment after the hot rolling and intermediateannealing in the middle of the cold rolling path. Thus, the cold-rolledsheet was obtained, which had a thickness of 2.0 mm.

Furthermore, the respective cold-rolled sheets in the examples weresubjected to a tempering treatment (T4) under conditions common to theexamples in heat treatment facilities. Specifically, the sheets wereeach subjected to solutionizing treatment at 550° C. for a holdingperiod of 30 seconds. At this time, the average heating rate up to thesolutionizing treatment temperature was set to 50° C./second. After thesolutionizing treatment, at an average cooling rate of 30° C./second,the workpiece was subjected to water quenching down to room temperature.Just after the quenching, the workpiece was subjected to pre-agingtreatment at 100° C. for a holding period of 5 hours. After thepre-aging treatment, the workpiece was gradually (naturally) cooled toyield a T4 material.

In each of the examples, from the T4 material, a #5 tensile testspecimen (25 mm×25 mm GL×sheet thickness) according to a JIS Z 2201 wascollected. In order to simulate the addition of strain to the T4material when the material is formed into the structural part concerned,one out of variously changed pre strains was added to the #5 testspecimen in a tensile test that will be detailed later. The strain-added#5 test specimen was artificially aged under conditions shown in Table 2to make a tensile test. Thereafter, from this test specimen, sheet-formtest specimens each having a required size were cut out, and thenevaluated about the solute amount of Cu therein, and the dislocationdensity and the shock absorption performance thereof as follows:

Measurement of Solute Amount of Cu:

In a measurement of the solute amount of Cu, one of the above-mentionedsheet-form test specimens, which was a target to be measured, wasdissolved by a residue extracting method with hot phenol. The resultantsolid and solution were filtrated and separated from each other througha filter having a mesh (particle catching diameter) of 0.1 μm. The Cucontent by percentage in the separated solution was measured as thesolute Cu amount.

This residue extracting method with hot phenol was specificallyperformed as follows: Initially, phenol was put into a decomposing flaskand then heated. The sheet-form test specimen to be measured wastransferred into the decomposing flask to be heated and decomposed.Next, benzyl alcohol was added thereto, and then the content wasfiltrated under reduced pressure to be separated into a solid and asolution. The Cu content by percentage in the separated solution wasquantitatively determined.

This quantitative determination appropriately made use of, e.g., atomicabsorption spectrophotometry (AAS), or inductively coupled plasmaemission spectrometry (ICP-OES). As described above, for the filtrationunder reduced pressure, a membrane filter was used which had a mesh of0.1 μm and a diameter of 47 mm.

The measurement and calculation were made about three samples collectedfrom arbitrarily-selected three sites of the sheet-form test specimen.The respective solute amounts (% by mass) of Cu in these samples wereaveraged and the resultant value was defined as the solute Cu amount.

Measurement of Dislocation Density:

A surface of one of the sheet-form test specimens was caused to simulatea surface of the structural part concerned, and the dislocation density(×10¹⁴ m⁻²) of the sheet-form test specimen surface was measured byX-ray diffraction under the above-mentioned conditions. The measurementwas made about arbitrarily-selected 5 sites of the sheet-form testspecimen. The respective dislocation densities of these sites wereaveraged and the resultant value was defined as the average dislocationdensity (×10¹⁴ m⁻²).

Tensile Test:

The artificially-aged #5 tensile test specimen was used to make atensile test at room temperature. At this time, the tensile direction ofthe test specimen was made parallel to the rolling direction. The testwas made at room temperature, 20° C., on the basis of JIS Z 2241 (1980)at a supporting point distance of 50 mm and a constant tensile speed of5 mm/minute until the test specimen was broken. If the test specimen hada 0.2% yield strength of 250 MPa or more, the specimen was judged to beacceptable as an artificially-aged structural part.

Shock Absorption Performance:

A bending test for evaluating shock absorption performance was made inaccordance with the following VDA bending test: “VDA 238-100 platebending test for metallic materials”, which is standardized by Verbandder Automobilindustrie e.V. (VDA). This test method is illustrated inFIG. 1 as a perspective view.

As represented by dot lines in FIG. 1, initially, one of the sheet-formtest specimens is put onto two rolls arranged in parallel to each otherand having a roll gap therebetween, so as to make right and left partsof the specimen equal in length to each other and be horizontallystretched.

Specifically, the sheet-form test specimen is put onto the two rolls, soas to make right and left parts of the specimen equal in length to eachother and be horizontally stretched in such a manner that the rollingdirection of the specimen is made perpendicular to the extendingdirection of a plate-form pushing/bending member arranged to standupward and vertically, and that the center of the specimen is positionedat the center of the narrow roll gap.

The pushing/bending member is pushed from the above onto the center ofthe sheet-form test specimen to apply a load thereto. In this way, thissheet-form test specimen is pushed (thrusted) toward the narrow roll gapto be bent. Thus, the bent and deformed center of the sheet-form testspecimen is pushed into the narrow roll gap.

When the load F from the above through the pushing/bending member turnsmaximum in this case, the angle of the outside of the bent center of thesheet-form test specimen is measured as the bending angle (°) of thespecimen. In accordance with the value of the bending angle, the shockabsorbance performance is evaluated. As this bending angle is larger,the sheet-form test specimen is higher in shock absorbance performance(crash performance) to continue to have the bending deformation withoutbeing collapsed in the middle.

Test conditions of this VDA bending test are shown hereinafter, usingsymbols described in FIG. 1. The sheet-form test specimen is made into asquare shape having a width “b” of 60 mm and a length “1” of 60 mm. Thediameter D of each of the two rolls is set to 30 mm; and the roll gap L,to 4 mm, which is two times the sheet thickness of the sheet-form testspecimen. The symbol S represents the pushed-depth of the center of thesheet-form test specimen into the roll gap when the load F becomesmaximum.

As illustrated in FIG. 1, about the plate-form pushing/bending member, aside of the member at the low-end-side thereof, which is to be pushedonto the center of the sheet-form test specimen, is made into a taperedform such that the tip (lower end) thereof has a radius of 0.2 mm.

In each of the examples, the VDA bending test was made about three ofthe sheet-form test specimens (made three times). The average thereofwas used as the bending angle (°) of the example. The results are shownin Table 2.

As is evident from Table 2, the above-specified preferred strain orartificial aging treatment is applied to each of Invention Examples 1 to13, which make use of respective aluminum alloys represented by alloynumbers 1 to 10 (within the composition range in the present invention)in Table 1.

Thus, these examples are in an artificially aged sheet state whichsimulates the structural part concerned, and satisfy the averagedislocation density specified in the invention.

As a result, also about the crash performance evaluated in the VDAbending test, the bending angle is 90° or more to satisfy an excellentproperty required for the structural part. Moreover, the 0.2% yieldstrength thereof is also a high strength of 250 MPa or more to satisfy aproperty required for the structural part.

In contrast, about each of the comparative examples, the alloycomposition thereof is out of the range in the present invention, or thestrain requirement thereof is out of the preferred range although thealloy composition is within the range in the invention.

Thus, each of the comparative examples does not satisfy the averagedislocation density specified in the invention.

As a result, about the comparative example, the 0.2% yield strength orthe crash performance evaluated in the VDA bending test is poorer thanthat about the invention examples not to satisfy a property required forthe structural part.

About Comparative Examples 14 to 20, the alloy composition is within therange in the present invention as shown about alloy number 2 or 5 inTable 1, but producing conditions for their raw sheet are out of thepreferred producing-conditions, or the preferred pre-strain is never orinsufficiently added to the structural part that has not yet beenartificially aged. Thus, on the whole, the average dislocation densityrange thereof is downwards out of the range specified in the presentinvention.

Comparative Example 14 does not satisfy the average dislocation densityspecified in the present invention since the lowest temperature in thehot rough rolling is too low in the production of the raw sheet.

About Comparative Example 15, the average dislocation density rangethereof is downwards out of the range specified in the present inventionbecause the average cooling rate is too small from the material (sheet)temperature just after the hot finish rolling to a material temperatureof 150° C. in the production of the raw sheet.

About Comparative Examples 16 and 17, the pre-strain is never orinsufficiently added thereto.

About Comparative Example 18, the added pre-strain is too large.

About Comparative Examples 19 and 20, the artificial aging treatmenttemperature is too high or the holding period is too long relatively tothe added pre-strain.

About Comparative Examples 21 and 22, their raw sheet is produced underthe above-mentioned preferred conditions, and the strain is addedthereto under the preferred conditions. However, their alloy compositionis downwards out of the range about the Mg or Si content by percentagein the present invention, as shown about the alloy numbers 11 and 12 inTable 1.

Thus, their average dislocation density range is downwards out of therange specified in the invention, so that the BH response is remarkablylowered and the strength and the crash performance are too low.

The above-mentioned results support respective critical significances ofthe requirements of the present invention for causing the aluminum alloystructural part of the present invention to have both of a crashperformance estimated in the VDA bending test, and a high strength.

TABLE 1 6000 Series aluminum alloy chemical-component-composition (% bymass) (balance: Al) Alloy No. Mg Si Fe Cu Me Zr Cr Ag Sn Sc Ti 1 0.580.95 0.17 — — — — — — — — 2 0.45 1.1 0.15 — — — — — — — 0.02 3 0.35 1.20.08 — — — — — — — 0.02 4 0.95 0.54 — — 0.35 — — — — — 0.02 5 0.43 1.1 —0.08 0.12 — — — — — — 6 0.60 1.0 — 0.72 — — — — — — 0.02 7 1.0 1.4 0.20— 0.08 — 0.12 — — — — 8 0.70 0.95 0.12 0.18 — 0.15 0.05 — — — 0.02 9 1.41.1 0.14 — — — — 0.15 — 0.07 0.02 10 0.45 0.66 0.15 0.45 0.07 — 0.05 —0.05 — 0.02 11 0.22 0.88 0.14 — 0.08 0.05 — — — — 0.02 12 0.75 0.43 —0.12 0.08 — — — — — 0.02

TABLE 2 Producing method Microstructure and properties after pre-stainHot finish rolling addition and artificial aging treatment (T6) Hotrough Average cooling rate Artificial aging Microstructure Propertiesrolling (° C./hr.) treatment Average 0.2% VDA Lowest from material Pre-End-point Holding dislocation Solute Cu Yield bending No. in temperaturetemperature just after strain temperature period density qualitystrength angle Classification No. Table 1 (° C.) rolling-end to 150° C.% (° C.) (min) (×10¹⁴ m⁻²) (% by mass) (MPa) (°) Invention 1 1 480 10 10210 30 4.5 — 253 98 Examples 2 2 470 7 5 250 10 3.4 — 255 107 3 2 470 810 230 20 4.6 — 258 101 4 2 470 7 15 230 15 5.6 — 265 94 5 2 460 10 20200 10 7.3 — 276 90 6 3 460 7 10 230 20 4.1 — 257 103 7 4 470 6 20 270 55.2 — 254 95 8 5 460 10 10 230 20 5.2 0.07 265 96 9 6 460 15 10 230 206.7 0.63 280 97 10 7 450 7 15 230 20 7.6 — 277 90 11 8 460 10 10 230 206.0 0.16 272 98 12 9 450 7 5 210 20 6.2 — 270 92 13 10 470 15 10 250 203.1 0.42 254 110 Comparative 14 2 430 7 5 250 10 2.7 — 244 88 Examples15 5 460 4 10 250 20 2.9 0.03 248 85 16 2 470 7 — 230 20 1.5 — 228 98 172 470 7 2 230 20 2.4 — 239 93 18 2 470 7 25 230 20 8.5 — 290 75 19 5 46010 10 280 15 2.6 0.03 243 89 20 5 460 10 10 230 40 2.8 0.04 247 88 21 11480 10 10 210 30 2.7 — 242 88 22 12 480 10 10 210 30 2.8 0.11 245 86

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide a structural partobtained from a 6000 series aluminum alloy sheet as a shaping rawmaterial, and having an improved crash performance; and a method forproducing the sheet. The present invention is therefore suitable for astructural part contributing to weight saving for, e.g., an automobile,a bicycle or a railroad vehicle.

1. An aluminum alloy structural part excellent in crash performance,comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percent symbolseach representing % by mass, and Al and inevitable impurities as thebalance of the part; and the structural part having an averagedislocation density of 3.0×10¹⁴ to 8.0×10¹⁴ m⁻², the density beingmeasured by X-ray diffraction of the surface.
 2. The aluminum alloystructural part excellent in crash performance according to claim 1,further comprising Cu: 0.05 to 1.0%, the percent symbol representing %by mass; and the amount of solute Cu in a solution separated from thestructural part by a residue extracting method with hot phenol beingfrom 0.05 to 1.0% by mass of the solution.
 3. The aluminum alloystructural part excellent in crash performance according to claim 1,further comprising one or more of the following: Mn: 0.05 to 0.5%, Zr:0.02 to 0.20%, and Cr: 0.02 to 0.15%, the percent symbols eachrepresenting % by mass.
 4. The aluminum alloy structural part excellentin crash performance according to claim 1, further comprising one ormore of the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02to 0.1%, the percent symbols each representing % by mass.
 5. A methodfor producing an aluminum alloy structural part excellent in crashperformance, comprising: applying homogenization to an aluminum alloyingot comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percentsymbols each representing % by mass, and Al and inevitable impurities asthe balance of the ingot, and subsequently rolling the ingot into asheet; subjecting the sheet further to solutionizing and quenchingtreatments, and subsequently cold-working the treated sheet to be formedinto a structural part while giving a strain of 5 to 20% to the sheet;thereby adjusting the artificially aged structural part to have adislocation density of 3.0×10¹⁴ to 8.0×10¹⁴ m⁻², the density beingmeasured by X-ray diffraction of the surface.
 6. The method forproducing an aluminum alloy structural part excellent in crashperformance according to claim 5, wherein the aluminum alloy structuralpart further comprises Cu: 0.05 to 1.0%, the percent symbol representing% by mass; and the amount of solute Cu in a solution separated from thestructural part by a residue extracting method with hot phenol is from0.05 to 1.0% of the solution.
 7. The method for producing an aluminumalloy structural part excellent in crash performance according to claim5, wherein the aluminum alloy structural part further comprises one ormore of the following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02to 0.15%, the percent symbols each representing % by mass.
 8. The methodfor producing an aluminum alloy structural part excellent in crashperformance according to claim 5, wherein the aluminum alloy structuralpart further comprises one or more of the following: Ag: 0.01 to 0.2%,Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%, the percent symbols eachrepresenting % by mass.
 9. The method for producing an aluminum alloystructural part excellent in crash performance according to claim 5,wherein the strain is given to the sheet when the sheet is formed intothe structural part.
 10. An aluminum alloy sheet excellent in crashperformance, for a structural part, comprising Mg: 0.30 to 1.5%, and Si:0.50 to 1.5%, the percent symbols each representing % by mass, and Aland inevitable impurities as the balance of the sheet; and the followingsheet having, as a microstructure, an average dislocation density of3.0×10¹⁴ to 8.0×10¹⁴ m⁻², the density being measured by X-raydiffraction of the surface: a surface of the sheet which is obtained,for simulating use of the structural part, by subjecting the sheet tosolutionizing treatment of keeping the sheet at 550° C. for 30 seconds,water quenching the sheet immediately down to room temperature at anaverage cooing rate of 30° C./s, subjecting the sheet, immediately afterthe quenching, to a pre-aging treatment at 100° C. for 5 hours, giving astrain of 10%, after the treatment, to the sheet through a tensiletester and further aging the sheet artificially at 210° C. for 30minutes.
 11. The aluminum alloy sheet excellent in crash performanceaccording to claim 10, further comprising Cu: 0.05 to 1.0%, the percentsymbol representing % by mass; and the amount of solute Cu in a solutionseparated from the aluminum alloy sheet by a residue extracting methodwith hot phenol being from 0.05 to 1.0% by mass of the solution.
 12. Thealuminum alloy sheet excellent in crash performance according to claim10, further comprising one or more of the following: Mn: 0.05 to 0.5%,Zr: 0.02 to 0.20%, and Cr: 0.02 to 0.15%, the percent symbols eachrepresenting % by mass.
 13. The aluminum alloy sheet excellent in crashperformance according to claim 10, comprising one or more of thefollowing: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%,the percent symbols each representing % by mass.